1. Field of the Invention
The present invention generally relates to a direct fuel injection internal combustion engine that employs a center injection arrangement in which a fuel injection valve and a spark plug are arranged on or near a reciprocation axis of the piston. More specifically, the present invention relates to an improvement to the shapes of cavities located in a top surface of the piston and to the control of frequency and timing of the fuel injection.
2. Background Information
An example of a direct fuel injection internal combustion engine that employs a center injection arrangement in which a fuel injection valve and spark plug are arranged on or near the reciprocation axis of the piston is disclosed in Japanese Laid-Open Patent Publication No. 2000-265841. In this publication, the top surface of the piston is provided with a deep dish part (an inner cavity) that is substantially axially symmetrical with respect to the reciprocation axis of the piston and a plurality of shallow dish parts (outer cavities) that are arranged discontinuously around the perimeter of the deep dish part. The shallow dish parts constitute a valve recess shape to avoid interfering with intake and exhaust valves and are not axially symmetrical with respect to the reciprocation axis of the piston. In this publication, the shape of a fuel stream injected from an injector is that of a hollow cone. When operating in a region of low to medium speed, the fuel injection timing is controlled to occur during a latter half of the compression stroke such that the fuel stream hits the deep dish part. When the fuel stream hits the deep dish part, a high stratified state is obtained in which an agglomerate fuel-air mixture forms inside the deep dish part and thereabove. When operating in a region of high speed, the fuel injection timing is controlled to occur during a former half of the compression stroke such that the fuel stream hits the shallow dish parts. When the fuel stream hits the shallow dish parts, the fuel stream is guided by lateral wall surfaces of the shallow dish parts such that it swirls upward. As a result, a low stratified state is obtained in which a fuel-air mixture agglomeration is formed inside the shallow dish parts and thereabove.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved direct fuel injection internal combustion engine. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that, in the direct fuel injection internal combustion engine of the prior art explained above, the fuel stream injected into the shallow dish parts spills into the spaces between the shallow dish parts. This spillage of the fuel stream creates the risk of degraded fuel efficiency and increased unburned hydrocarbons when the shallow dish parts are used to conduct stratified charge combustion. On the other hand, when the deep dish part is used to conduct stratified charge combustion, the fuel stream injected at the deep dish part easily spills into the shallow dish parts because the side wall of the deep dish part joins with the shallow dish parts in gradual slopes. Thus, when the deep dish part is used to conduct the stratified charge combustion, degraded fuel efficiency and increased unburned hydrocarbons are also a concern. Moreover, depending on the diameter of the shallow dish parts and the shape of the lateral wall surfaces of the shallow dish parts, the fuel-air mixture agglomeration may be formed in the shape of a donut and cause the stability of the spark plug ignition to be reduced. The upward-swirling fuel stream can be made to gather in the center of the combustion chamber by slanting the lateral wall surfaces inward (into a reentrant shape). However, the more the lateral wall surfaces are slanted, the poorer the S/V ratio of the combustion chamber becomes and the more the output and fuel efficiency performance are degraded. Furthermore, the act of gathering the upper part of the fuel-air mixture in the center of the combustion chamber is contrary to the original objective of obtaining a low stratified charge state.
The forgoing object of the present invention can basically be attained by providing a direct fuel injection internal combustion engine comprising a combustion chamber, a fuel injection valve and a spark plug. The combustion chamber has a piston movably mounted therein along a reciprocation axis. The fuel injection valve is arranged adjacent the reciprocation axis of the piston to inject a fuel stream directly into the combustion chamber. The spark plug is arranged adjacent the reciprocation axis of the piston to ignite a fuel-air mixture inside the combustion chamber. The piston includes an outer cavity located in a top surface of the piston and being substantially axially symmetrical about the reciprocation axis of the piston. The piston also includes an inner cavity located in the outer cavity and being substantially axially symmetrical about the reciprocation axis of the piston.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.